Impregnated cathode and method for manufacturing the same

ABSTRACT

An impregnated cathode whose initial electron emitting performance, lifetime property, and insulating property for an electron gun are excellent and that is suitable for mass production, and a method for manufacturing the same. In the impregnated cathode, the porosity of the sintered body of porous metal is continuously increased as the distance in the depth direction from an electron emitting face is increased. A pellet of sintered body of metal raw material has pores in it. The pores are filled with electron emitting material. The porosity is continuously increased as the distance in the depth direction from an electron emitting face is increased. Thus, since the discontinuity inside the pellet is not formed, a reaction generating free Ba continuously and smoothly proceeds on the entire pellet. In addition, since raw material powder having more than one kind of particle size is not necessary to be used, the manufacturing process can be simplified. Moreover, various functions such as lifetime property, etc. can be improved by making the porosity and porosity distribution in a certain range.

FIELD OF THE INVENTION

The present invention relates to an impregnated cathode used for anelectron tube and a method for manufacturing the same.

BACKGROUND OF THE INVENTION

An impregnated cathode has a basic structure in which pores of asintered body of porous metal (pellet) are impregnated with an electronemitting material. A method for manufacturing an impregnated cathodecomprises the steps of: press molding powder of a high melting pointmetal such as tungsten, etc.; then sintering the press molded product toform a reducing substrate having a proper porosity; and thenimpregnating the pores of the substrate with molten electron emittingmaterial comprising BaO, CaO and Al₂O₃ as the main components. Thus, acathode pellet is obtained. This cathode pellet is impregnated withemitting material in an amount corresponding to the volume of thesintered body and the porosity, i.e. the volume of pores.

The principle of operation of the cathode pellet will be explainedbelow. When the cathode pellet is subjected to a high temperatureactivation, BaO is reduced by the pellet to generate free Ba. This freeBa thermally diffuses in pores and reaches the surface of the pellet.Then, the free Ba thermally diffuses on the surface of the pellet, tothus form a Ba monoatomic layer on the surface of the pellet. At thistime, a monoatomic layer spreads to cover an area corresponding to thedifference between an amount of Ba evaporated from the monolayer, whichis dependent upon the temperature of the pellet, and an amount of Basupplied from the inside of the pellet. This Ba monoatomic layer reducesthe effective work function that is involved in an electron emissionfrom 4 to 5 eV of the metal itself constituting the pellet to about 2eV. Consequently, excellent thermionic emission is provided.

If little Ba is supplied from the inside of the pellet at the time ofthe operation, a necessary and sufficient area of Ba monoatomic layercannot be formed, causing a deficiency of emission. Moreover, therearise some problems, for example, the activation takes a long time, etc.

On the contrary, if too much Ba is supplied, Ba evaporated from thesurface of the pellet is increased, so that the BaO impregnated in thepellet is consumed in a short time and in turn the lifetime isshortened. Furthermore, the evaporated Ba is deposited on a counterelectrode, causing unnecessary electron emission, etc.

The most important point of the operation of the impregnated cathode isto form a necessary and sufficient Ba monoatomic layer in an early stageand to keep it for a long time. The factors for forming a Ba monoatomiclayer include: the amount of impregnated BaO; the reducing rate of theimpregnated BaO being reduced by the pellet; the thermal diffusionvelocity of free Ba in pores; and the surface thermal diffusion rate ofBa on an electron emitting face.

The design parameters for controlling the operations are: the amount ofimpregnation of electron emitting material; the porosity of the pelletand the spatial distribution of pores; and the cleanness of the electronemitting face, more specifically, an absence of extra electron emittingmaterial attached to the electron emitting face. The most importantthing for mass production is to control these parameters with highprecision and with less variation.

Based on the above mentioned background of the principle, Publication ofJapanese Patent Application (Tokko Sho) No. 44-10810 discloses animpregnated cathode, in which the evaporation of extra electron emittingmaterial can be inhibited, the leak of current in an insulating portionof an electron gun can be reduced, and an excellent state of Bamonoatomic layer can be maintained for a long time and in turn itslifetime can be extended.

The above mentioned structure is a two-layer structure comprising afirst layer having a low porosity on the side of the electron emittingface of the pellet, wherein the evaporation of the electron emittingmaterial is inhibited; and a second layer having a high porosity formedbelow the first layer. According to such a two-layer structure, evenafter the Ba supply capacity of the first layer is exhausted (i.e. afterthe lifetime), Ba can be supplied from the second layer to the firstlayer. Consequently, the lifetime of the pellet is further extended ascompared with the lifetime the first layer has naturally.

Furthermore, Publication of Japanese Patent Application (Tokkai Hei) No.6-103885 suggests that the surface roughness of the substrate be notmore than 5 μm, more preferably that the substrate be perfectly smooth,so as to easily remove the attached extra electron emitting materialafter impregnation.

Furthermore, Publication of Japanese Patent Application (Tokkai Sho) No.58-87735 discloses a manufacturing method in which compressed electronemitting materials placed on the upper surfaces of the individualpellets are melted and impregnated in order to ensure the amount ofimpregnation of the electron emitting material.

Furthermore, Publication of Japanese Patent Application (Tokkai Hei) No.6-103885 discloses a method of mass production in which the amount ofthe impregnated electron emitting materials is kept stable byclassifying metal raw material powder of the pellet and controlling theporosity of the pellet.

Furthermore, a mechanical method using a brush, a metal-clad needle,etc., a polishing method by means of cutting, etc., and ultrasoniccleaning in water, etc. have been conventionally suggested.

Furthermore, Publication of Japanese Patent Application (Tokkai Sho) No.50-103967 discloses a method in which a pellet is provided on thespecific jigs one by one and then washed by ultrasonic cleaning in cleanwater.

However, the above mentioned conventional impregnated cathodes have thefollowing problems.

(1) In order to manufacture the impregnated cathode having a two-layerstructure, it is necessary to use two different kinds of raw materialpowders or to carry out press molding twice. Consequently, theproduction process is complicated.

(2) In the method in which a pellet is treated one by one or the rawmaterial powder is classified, the productivity is poor and massproduction is difficult.

(3) The method of mechanically removing extra electron emittingmaterials by using a brush, metallic needle, etc., is difficult to carryout. Furthermore, a treatment is necessary for each pellet, so that massproduction is difficult.

(4) The manufacturing process in which the sintered pellets are providedon the specific jig one by one is complicated. It takes not less than 1hour to perfectly remove extra electron emitting materials by way ofonly the ultrasonic cleaning method. Consequently mass production isdifficult.

SUMMARY OF THE INVENTION

It is the object of the present invention to solve the above mentionedconventional problems and to provide an impregnated cathode and a methodof manufacturing the same, which is excellent in initial electronemitting performance, lifetime property, and insulating property andwhich is suitable for mass production by continuously increasing theporosity of the sintered body of porous metal as the distance in thedepth direction from the electron emitting face is increased.

In order to achieve the above mentioned objects, the first impregnatedcathode of the present invention has a cathode pellet in which the poreportion of a sintered body of porous metal is impregnated with electronemitting material, wherein the porosity of the sintered body of porousmetal is continuously increased as the distance in the depth directionfrom an electron emitting face is increased.

By the above mentioned impregnated cathode, since no discontinuity ofthe porosity in the pellet is formed, a reaction generating free Baproceeds continuously and smoothly all over the pellet. Moreover, sinceraw material powder having more than one kind of particle sizes need notbe used, the manufacturing process can be simplified.

It is preferable in the above mentioned first impregnated cathode thatthe porosity of an electron emitting face of the sintered body of porousmetal is in the range of 12.5 to 25 volume %; the porosity differencebetween the porosity of a vicinity of the electron emitting face and theporosity of a vicinity of the opposite face to the electron emittingface is in the range of 5 to 25 volume %; and the porosity of theopposite side to the electron emitting face is less than 40 volume %.With such an impregnated cathode, an excellent lifetime property can beobtained.

It is further preferable in the first impregnated cathode that thesurface roughness of the electron emitting face of the cathode pellet isin the range of 5 to 20 μm for the maximum height. With the abovementioned impregnated cathode, the emission property can be enhanced.

Next, according to a first method for manufacturing an impregnatedcathode of the present invention, a method for manufacturing animpregnated cathode having a cathode pellet in which the pore portion ofa sintered body of porous metal is impregnated with electron emittingmaterial, comprises the steps of press molding metal raw material powderto form a porous substrate, the press molding being conducted afterfilling the metal raw material powder in a struck-level cartridge andthen filling the raw material metal powder in a die by level strikingmeasurement; wherein a contacting face of the cartridge and the diesurface has an annular shape and the cartridge has an inclined face inwhich the end portion of the outside of the cartridge contacts with thedie surface.

According to the above mentioned manufacturing method, the levelstriking measurement can be conducted exactly, so that the particle sizedistribution of the raw material powder inside the cartridge can bereflected in the particle size distribution of the raw material to befilled in the press die. Consequently, the variation of the porosity ofthe pellet or manufacturing variation in the amount of impregnation ofelectron emitting materials can be reduced.

It is preferable in the first method for manufacturing an impregnatedcathode that the inner diameter of the annular shape is in the range of10 to 20 times as large as the diameter of a pellet; the externaldiameter of the annular shape is in the range of 1.05 to 1.3 times aslarge as the inner diameter; and the angle that the inclined face makeswith the die face is in the range of 40 to 80°

It is further preferable that an amount of metal raw material powderthat is filled in the cartridge is equal to an amount of 200 to 800cathode pellets.

It is further preferable that the metal raw material powder is heated attemperatures in the range of 50 to 100° C. at the time of level strikingmeasurement and pressing.

It is further preferable that a face at which a punch contacts withmetal raw material powder is referred to an electron emitting face; therelative descending speed of the punch to the die is in the range of 0.5to 5 cm/s, and the pressing time is in the range of 1 to 7 seconds whenthe punch contacts with metal raw material powder.

Next, according to the second method for manufacturing an impregnatedcathode of the present invention, a method for manufacturing animpregnated cathode having a cathode pellet in which the pore portion ofa sintered body of porous metal is impregnated with electron emittingmaterial comprises the steps of: press molding metal raw material powderto form a porous substrate; and sintering the porous substrate to form asintered body of porous metal; wherein the average porosity of theporous substrate after press molding is controlled by adjusting thepressure of press molding, and the average porosity of the sintered bodyof porous metal after sintering is controlled by adjusting the sinteringtemperature.

By the above mentioned method for manufacturing the impregnated cathode,it is not necessary to use raw material powder having a differentparticle sizes and to mold in multilayers. Consequently, the averageporosity of the entire pellet can be controlled by the general process.

It is preferable in the second method for manufacturing an impregnatedcathode that porosity distribution is controlled by adjusting thedescending speed of the punch and the pressing time. By the abovementioned method for manufacturing an impregnated cathode, it is notnecessary to use raw material powder having different particle sizes andto mold in multilayers. Consequently, the average porosity of the entirepellet can be controlled by general process.

Furthermore, it is preferable that an average porosity (D volume %) ofthe porous substrate after press molding and an average porosity (dvolume %) of the sintered body of porous metal after sintering has arelationship expressed by the following equation:

d+10≦D≦d+20.

By the above mentioned method for manufacturing an impregnated cathode,the pellets that ensures a certain amount of impregnation can bemanufactured by maintaining the mechanical strength and inhibiting thegeneration of closed pores.

Next, according to the third method for manufacturing an impregnatedcathode of the present invention, a method for manufacturing animpregnated cathode having a cathode pellet in which a pore portion of asintered body of porous metal is impregnated with electron emittingmaterial comprises the steps of placing the sintered body of porousmetal and the electron emitting material in a container for impregnationin such a manner that the electron emitting material contacts with anentire surface of the sintered body of porous metal when the electronemitting materials are melted, and impregnating the pore portion of thesintered body of porous metal with the electron emitting material.

With the above mentioned impregnated cathode, deficiency of impregnationcan be prevented. Consequently, stable impregnation can be obtained.

It is preferable in the third method for manufacturing an impregnatedcathode that electron emitting materials are filled in the container forimpregnation in such a manner that the depth of the electron emittingmaterials is uniform, and the sintered body of porous metal is locatedat the middle portion in the direction of the depth of the electronemitting material or located at the top of the electron emittingmaterial.

It is further preferable in the third method that the weight of theelectron emitting material to be filled in the container forimpregnation is in the range of 10 to 100 times as much as theimpregnatable weight of the sintered body of porous metal in thecontainer for impregnation. Herein, impregnatable weight means the totaleffective weight of emitting material that is carried by the poroussintered bodies, or something similar. By the above mentioned method formanufacturing an impregnated cathode, the variation of the amount ofimpregnation can be reduced.

It is further preferable in the third method that extra electronemitting materials are removed by shaking a container in which animpregnated cathode pellet and alumina ball are placed and washing byultrasonic cleaning in water. By the above mentioned method formanufacturing an impregnated cathode, extra electron emitting materialscan be removed while inhibiting the fracture rate of the pellet and thevariation of the amount of impregnation can be reduced.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual view of a cross section of an impregnated cathodeof one embodiment of the present invention.

FIG. 2 is a flow chart showing a manufacturing process of an impregnatedcathode for one embodiment of the present invention.

FIG. 3 is a sectional view of a press die and a cartridge for levelstriking measurement (a cartridge for striking the top surface of thepress die and the height of the raw material powder level) used for amethod for manufacturing an impregnated cathode of the presentinvention.

FIG. 4 is a graph showing the relationship between the porosity of anelectron emitting face and the saturation current and the relationshipbetween the porosity of an electron emitting face and the evaporatedamount of an impregnated cathode of one embodiment of the presentinvention.

FIG. 5 is a graph showing the relationship between the porositydifference and the lifetime of an impregnated cathode of one embodimentof the present invention.

FIG. 6 is a graph showing the relationship between the average porosityand the porosity difference of an impregnated cathode of one embodimentof the present invention.

FIG. 7 is a graph showing the relationship between the surface roughnessof an electron emitting face and the relative value of the saturationcurrent of an impregnated cathode of one embodiment of the presentinvention.

FIG. 8 is a graph showing the relationship between the filling amount ofmetal raw material powder and the variation of the weight of the pelletof an impregnated cathode of one embodiment of the present invention.

FIG. 9 is a graph showing the relationship between the heatingtemperature of the raw material powder and the variation of the weightof the pellet of an impregnated cathode of one embodiment of the presentinvention.

FIG. 10 is a graph showing the relationship between the average porosityof the porous substrate after press molding and the amount ofimpregnation of electron emitting material and the relationship betweenthe average porosity of the porous substrate after press molding and thefracture rate of the pellet of an impregnated cathode of one embodimentof the present invention.

FIG. 11 is a graph showing the relationship between the average porosityafter press molding and the average porosity after sintering of animpregnated cathode of one embodiment of the present invention.

FIG. 12 is a graph showing the relationship between the amount ofelectron emitting material filled in a container for impregnation andthe variation of the amount of impregnation to the pellet.

FIG. 13(A) is a graph showing the relationship between the location ofthe pellets at the time of impregnation and the amount of impregnationto the pellet of an impregnated cathode of one embodiment of the presentinvention.

FIG. 13(B) shows each location of the pellets in the container forimpregnation.

FIG. 14 is a graph showing the relationship between the shaking time andthe amount of impregnation to the pellet of an impregnated cathode ofone embodiment of the present invention and a comparative Example.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, one embodiment of the present invention will be explainedwith reference to the drawings.

Embodiment 1

FIG. 1 is a conceptual view of a cross section of an impregnated cathodepellet of Embodiment 1 of the present invention. The pellet of thisembodiment is a compressed sintered body of metal raw material powder 1.The pellet has pores in it, and the pores are filled with electronemitting materials 2. Arrow 4 illustrates the direction of the electronemission. Porosity is continuously increased along the direction from anelectron emitting face 3 to the side opposite to the electron emittingface (the direction expressed by arrow 5). Moreover, the surfaceroughness A (maximum height) of the electron emitting face 3 ismaintained in the range of 5 to 20 μm.

FIG. 2 is a flow chart showing a method for manufacturing an impregnatedcathode of Embodiment 1. In the process, metal raw material powder ispress molded after level striking measurement. The “level strikingmeasurement” means a measurement of the predetermined amount of rawmaterial that is accurately filled in a container by first heaping upthe raw material in the container and then striking the raw materiallevel along the edge of the container. The press molded product issintered in hydrogen or under vacuum at a temperature of 1500 to 2200°C. When the sintered body is heated along with electron emittingmaterials at the temperature of 1500 to 1800° C., electron emittingmaterials are melted and impregnated in the pores inside the pellet.Then extra electron emitting materials attached to the pellet areremoved. Thus, a pellet is completed by way of a surface coatingprocess.

Hereinafter, one example of the method for manufacturing the impregnatedcathode of Embodiment 1 will be explained in detail. First, a levelstriking measurement of raw material powder was carried out. FIG. 3shows a cartridge for striking the upper surface of metal raw materialpowder and the die level (hereinafter “cartridge” will be used for anabbreviation) and a press die used in the method for manufacturing animpregnated cathode of this embodiment. As a raw material for a poroussubstrate, tungsten powder having a particle size of 1 to 10 μm wasused. 3.5 g of raw material powder 7 was filled in a cartridge 6 on thesurface 9 a of the press die. This amount is equal to an amount of about500 pellets.

The face 10 of the struck level of the cartridge 6 had an annular shapehaving an inner diameter of 20 mm and an outer diameter of 22 mm, andhad an angle B, which the external side face 11 of the cartridge 5 makeswith the surface 9 a of the press die, of 60°. Level measurement wascarried out 2 to 6 times while heating the raw material powder 7 atabout 80° C. by means of a heater, and 7 mg of raw material powder 7 wasfilled in a through hole portion 9 of the press die. Next, press moldingwas carried out with a common punch 8. The descending speed of the punch8 was controlled to 1 cm/s, and the pressing time was 4 seconds.

In order to make the average porosity of the sintered pellet 20% at thetemperature in the range of 1850 to 2000° C., press load was controlledin the range of 2×10⁸ to 10×10⁸ N/m² so that the average porosity afterpress molding was about 35%.

In the following sintering step, sintering was conducted in reducingatmosphere for about 2 hours. The porosity of the pellet manufactured byway of the above mentioned steps was 17 volume % (vol. %) in theelectron emitting face that contacts with the punch, 23 vol. % in theopposite side to the electron emitting face and the average of theseporosities was 20 vol. %. Moreover, as to the surface roughness of theelectron emitting face 3, the maximum height was in the range of 5 to 10μm.

Furthermore, the average porosity can be controlled by adjusting thepress load and sintering temperature. The spatial distribution of theporosity can be controlled by adjusting the descending speed of thepunch and pressing time.

Herein, the porosity and the method for evaluating the porosity areexplained. The porosity can be calculated by the following equation, bymeasuring volume V (cm³) and weight W (g); and using a bulk density ofmetal raw material p (g/cm³).

Porosity of the pellet (vol. %)=[(V−W/ρ)/V]×100

Moreover, the porosity distribution in the pellet can be evaluated bythe following equations by using d1, d2 and d3. The d1, d2 and d3 denotethe average porosity of each of the sectional portions obtained bydividing the pellet into three parts. Therein, these parts are obtainedby cutting the pellet with a cut face parallel to the electron emittingface in the direction perpendicular to the electron emitting face.

Porosity of the electron emitting face=d1−(d2−d1)/2

Porosity of the opposite side=d3+(d3−d2)/2

Herein, d1 denotes an average porosity of the sectional portion at theside of the electron emitting face among the three divided portions ofthe pellet; d2 denotes an average porosity of the sectional portions inthe middle portion among the three divided portions of the pellet; andd3 denotes an average porosity of the sectional portion at the sideopposite to the electron emitting face among the three divided portionsof the pellet.

Herein, the dividing number is not limited to 3. It may be 2 and also 4or more. The porosity distribution can be evaluated mathematically bycalculating by the above mentioned equations.

Next, the impregnation of electron emitting material was carried out. Amixture comprising BaCO₃, CaCO₃, and Al₂O₃, in the mole ratio of 4:1:1was used as electron emitting material. The electron emitting materialsare filled in a cylindrical container for impregnation having a diameterof about 1.5 cm and a depth of about 1 cm. The filled weight of theelectron emitting material was about 30 times as much as the weight ofthat to be impregnated in the porous substrate. 100 sintered poroussubstrates were provided with the electron emitting materials.

The container for impregnation was placed in a furnace at thetemperature of 1500 to 1800° C. in reducing atmosphere. Consequently,the porous substrate was impregnated with the molten electron emittingmaterials. Moreover, since BaCO₃ and CaCO₃ are previously decomposedinto oxides BaO and CaO respectively, the pellet is impregnated withthese oxides.

Next, extra electron emitting materials attached to the surface of theporous substrate were removed. This removal was carried out as follows:the impregnated pellet was placed in a small container along with sixalumina balls having a diameter of φ5 mm and shaken for about 5 minutes.Then, the impregnated pellet was cleaned by ultrasonic cleaning in waterfor about 5 minutes and finally dried, and thus the pellet wascompleted.

In addition, Os thin film was formed on the electron emitting face ofthe manufactured porous substrate, i.e. the side contacting with presspunch by the sputtering method. The cathode was completed by way of theabove mentioned steps. This cathode is incorporated into, for example,the electron gun of a 17″ cathode ray tube. This cathode can have acurrent density of 2 to 4 A/cm² as the continuous electron emitting onperformance at the normal operation temperature of 1000° C. And thecathode has several tens of hundreds hours for an emission lifetime.

In the above mentioned pellet of the present invention, a face ofdiscontinuity of the porosity was not formed in the pellet.Consequently, a chemical reaction generating free Ba proceedscontinuously and smoothly on the entire pellet. Furthermore, since it isnot necessary to use raw material powder having more than one particlesize distribution, it can provide a manufacturing method that issimplified and that is suitable for mass production.

Embodiment 2

In Embodiment 2, the porosity and the porosity distribution of thepellet manufactured by the method of Embodiment 1 were conducted forcertain ranges. Various kinds of pellets were manufactured in themanufacturing process explained in Embodiment 1, wherein the porosity ofthe electron emitting face and the porosity difference between theporosity of the electron emitting face and the porosity of the oppositeface (“porosity difference” will be used hereinafter) were varied. Thesepellet were completed as cathodes and incorporated into the commerciallyavailable 17″ cathode ray tube for monitoring. A forced accelerated lifetest was conducted at the cathode operation temperature of 1250° C.while 400 μA of direct current per cathode was taken out as an emission.

The measurement results of an initial saturation emission current of theabove mentioned various kinds of pellets (“saturation current” will beused hereinafter), an initial amount of evaporation of the electronemitting materials per unit time (“evaporation amount” will be usedhereinafter), and an emission lifetime (“lifetime” will be usedhereinafter) are shown in Table 1. In Table 1, the values of thesaturation current, evaporation amount and lifetime are relative values,with the respective measurement values being 1 when the porosity of theelectron emitting face was 20 vol. % and the porosity difference was 0.

Furthermore, FIG. 4 is a graph showing the relationship between theporosity of an electron emitting face and the saturation current and therelationship between the porosity of an electron emitting face and theevaporation amount. Similarly, FIG. 5 is a graph showing therelationship between the porosity difference and the lifetime.

TABLE 1 Porosity of an electron Evalu- Porosity difference between theopposite side and emitting face ation the side of an electron emittingface (vol. %) (vol. %) Items 0 5 10 15 20 25 30 10 A 0.65 0.65 0.65 0.650.65 0.65 0.65 B 0.5 0.5 0.5 0.5 0.5 0.5 0.5 C 1.2 1.4 1.5 1.6 1.7 1.71.5 D 10 12.5 15 17.5 20 22.5 25 12.5 A 0.75 0.75 0.75 0.75 0.75 0.750.75 B 0.6 0.6 0.6 0.6 0.6 0.6 0.6 C 1.15 1.4 1.45 1.5 1.45 1.4 1.2 D12.5 15 17.5 20 22.5 25 27.5 15 A 0.85 0.85 0.85 0.85 0.85 0.85 0.85 B0.75 0.75 0.75 0.75 0.75 0.75 0.75 C 1.1 1.35 1.4 1.45 1.4 1.25 0.8 D 1517.5 20 22.5 25 27.5 30 20 A 1 1 1 1 1 1 1 B 1 1 1 1 1 1 1 C 1 1.2 1.31.35 1.3 1.15 0.8 D 20 22.5 25 27.5 30 32.5 35 25 A 1.1 1.1 1.1 1.1 1.11.1 1.1 B 1.25 1.25 1.25 1.25 1.25 1.25 1.25 C 0.9 1.1 1.25 1.3 1.251.05 0.6 D 25 27.5 30 32.5 35 37.5 40 30 A 1.15 1.15 1.15 1.15 1.15 1.151.15 B 1.5 1.5 1.5 1.5 1.5 1.5 1.5 C 0.6 0.7 0.8 0.6 0.4 0.3 0.2 D 3032.5 35 37.5 40 42.5 45 A: saturation current B: amount of evaporationC: lifetime D: average porosity

Table 1, and FIGS. 4 and 5 show the following things.

(i) If the porosity of the electron emitting face is kept constant, thesaturation current and amount of evaporation are constant regardless ofthe average porosity.

(ii) If the porosity of the electron emitting face is varied, as shownin FIG. 4, the saturation current is slowly increased in accordance withthe increase of the porosity of the electron emitting face and saturatedwhen the porosity of the electron emitting face is around 30 vol. %.

(iii) On the other hand, the evaporation amount is increasedapproximately in proportion with the porosity of the electron emittingface, so that when the porosity of the electron emitting face isincreased more than the predetermined value, unnecessary electronemission may be increased at the electrode of the electron gun.Therefore, in practice, it is necessary to compromise the saturationcurrent and amount of evaporation. More specifically, it is preferablethat the porosity of the electron emitting face is in the range of 12.5to 25 vol. %.

(iv) As shown in FIG. 5 and Table 1, when the porosity difference is inthe range of 5 to 25 vol. %, the lifetime is extended in the range of 10to 40% as compared with the lifetime where there is no porositydifference.

Moreover, not shown in Table 1, when the porosity of the side oppositeto the electron emitting face is not less than 40 vol. %, the mechanicalstrength of the pellet is weakened. Therefore, it is preferable inpractice that the porosity of the opposite side to the electron emittingface be less than 40 vol. %.

According to above mentioned results, the effective choice of theporosity and porosity distribution: in the range of 12.5 to 25 vol. %for the porosity of the electron emitting face; in the range of 5 to 25vol. % for the porosity difference; and less than 40% for the porosityof the side opposite to the electron emitting face.

The above mentioned effective choice can be expressed as follows:

15≦ρ≦30  (Equation 1)

5≦Δρ≦25  (Equation 2)

 Δρ<2×(40−ρ)  (Equation 3)

Δρ≦2×(ρ−12.5)  (Equation 4)

wherein the average porosity is ρ vol. % and the porosity difference isΔρ vol. %.

The lower limit value of Equation 1, 15 vol. % was determined from thefact that the lower limit value in the preferable range of the porosityof the electron emitting face was 12.5 vol. % and the lower limit valueof the preferable range of the porosity difference was 5. vol. %. Theupper limit of the Equation 1, 30 vol. % was determined as the maximumvalue at Table 1, which satisfied the below mentioned conditions wherethe upper limit value in the preferable range of the porosity of theelectron emitting face was 25 vol. % and the porosity of the oppositeside to the electron emitting face was less than 40 vol. %.

Equation 3 was determined from the condition where the porosity of theopposite side to the electron emitting face was less than 40 vol. %.Equation 4 was determined from the condition where the porosity of theelectron emitting face was not less than 12.5 vol. %.

FIG. 6 shows the relationship of Equations 1 to 4. The hatched portionof FIG. 6 shows the range satisfying Equations 1 to 4. In other words,if the average porosity p and the porosity different Δρ of the pelletare selected from the hatched portion of FIG. 6, an excellent lifetimeproperty can be obtained. Furthermore, when the necessary emission andamount of evaporation are selected from this range, the best pelletdesign is possible.

Embodiment 3

In Embodiment 3, the emission property was enhanced by forming a certainrange of surface roughness on the electron emitting face of the pellet.FIG. 7 shows the relationship between the surface roughness and therelative value of the saturation current. The saturation current wasmeasured by making a trial manufacture of the pellet as a usual cathode.The relative values shown by the vertical axis of FIG. 7 are expressedbased on the value of 1 at the pellet having a surface roughness of theelectron emitting face of 0 μm.

The horizontal axis of FIG. 7 shows the surface roughness of theelectron emitting face of the pellet. The measurement was conducted forfour kinds of pellets classified based on the range of the surfaceroughness. More specifically, the range of the surface roughness at thepoints a to d are: 0 to 5 μm for point a; 5 to 10 μm for the point b; 10to 20 μm for the point c; and 20 to 30 μm for the point d. The surfaceroughness represents the maximum height.

FIG. 7 shows that as the surface roughness is increased, the relativevalue of the saturation current is increased and the pellet becomes moreexcellent. The relative values of the saturation current at any of thepoints b, c, and d are not less than 1. However, at the point d, sparkswere found to be generated between the facing anode in some cases (thepoint e of FIG. 7). Therefore, the points b and c of FIG. 7 arepreferred. In other words, from the viewpoint of inhibiting sparks andmaximizing the emission, it is preferred that the surface roughness isin the range of 5 to 20 μm.

Moreover, in the above mentioned measurement, the pellet having aporosity of the electron emitting face of 17 vol. %, and porositydifference of 6 vol. % was used. However, if the pellet having the othervalues is used, the relationship between the surface roughness andsaturation current is similar. It is preferable that the surfaceroughness is in the range of 5 to 20 μm.

Furthermore, since the pellet that is manufactured by the basic processexplained in Embodiment 1 has the surface roughness of 5 to 10 μm, itssurface was mechanically abraded to form the pellet having a surfaceroughness of 0 to 5 μm. Moreover, the pellet having a surface roughnessof 10 to 30 u m was manufactured by sintering by attaching tungstenpowder of about 10 to 20 μm to the surface of the substrate after pressmolding.

Embodiment 4

The most important thing for mass production of cathode pellets is toreduce the variation of the porosity per pellet and to stabilize theamount of electron emitting materials. In the basic process explained inEmbodiment 1, the embodiments for reducing the variation inmanufacturing will be explained with reference to the followingEmbodiments 4 to 11.

Embodiment 4 refers to the shape of the cartridge used for the pressmolding process. The optimum shape of the cartridge of Embodiment 4 willbe explained. It is important for a cartridge 6 to precisely reflect theparticle distribution of the raw material powder 7 on the particle sizedistribution of raw material powder to be filled in the press die.

Therefore, the shape and size of the contacting surface 10 between thecartridge 6 and the surface 9 a of the press die are important. Morespecifically, it is preferable that the shape of the contacting surface10 is an annular shape. If the shape is an annular shape, in thereciprocating motion of striking level, stirring of raw material powdercan be conducted in the cartridge 6.

If the shape of the contacting face is square, even if the reciprocatingmotion is conducted, the two dimensional stirring of powder in thehorizontal direction of the press die cannot be expected. If thecartridge 6 is set in such a manner that diagonal lines of square shape,etc. is made to pass the through hole 9, two dimensional stirring can beexpected. In this case, however, since the corner portions of thecartridge 6 contact with the end portion of the through hole 9, thecartridge 6 and press die are damaged.

It is preferable that in a case where the contacting face 10 is annularin shape, the inner diameter of the circle is 10 to 20 times as large asthe inner diameter of the through hole 9 (the diameter of the pellet).If the inner diameter of the circle is less than 10 times as large asthe inner diameter of the through hole 9, stirring effect of powder islowered. As a result, a pellet whose particle distribution becomesrougher as pressing is conducted is manufactured. Moreover, if the innerdiameter of the circle is more than 20 times as large as the innerdiameter of the through hole, the stirring effect is further enhanced,but a stroke of the reciprocating motion is longer. Consequently, themass production capability is deteriorated.

It is preferable that the outer diameter of the circle is in the rangeof 1.05 to 1.3 times as large as the inner diameter. If the outerdiameter is less than 1.05 times as large as the inner diameter,one-sided reduction occurs due to its contacting with press die, so thatthe cartridge cannot be used for a long time. Moreover, if the outerdiameter is more than 1.3 times as large as the inner diameter, theadhesiveness between the annular portion and the surface 9 a of thepress die is poor, so that the level striking measurement cannot exactlybe conducted. In addition, fine powder can enter the gap of thecontacting face 10, so that level striking measurement cannot beconducted.

An external face 11 of the cartridge that contacts with the outerdiameter of the circle is preferably an inclined face. An angle B thatthe external face 11 makes with the contacting face is preferably in therange of 40 to 80°. If the angle is less than 40°, raw material powdersare involved at the time of the level striking operation, so thatmeasurement sometimes becomes inaccurate. On the other hand, if theangle is more than 80°, raw material powders are held at the time ofcontacting the end portion of the though hole 9 and the cartridge 6, sothat a smooth level striking operation cannot be conducted.

Embodiment 5

Embodiment 5 refers to a manufacturing method in which an amount ofmetal raw material powder filled in the cartridge of metal is made to bein the certain range of amount. FIG. 8 shows the relationship between afilling amount of metal raw material powder and the variation of thepellet weight. In order to obtain the measurement results of FIG. 8, thepellet was manufactured by varying the filling amount of the tungstenpowder from the an amount corresponding to the weight of 100 pellets(about 0.7 g) to the weight of 2000 pellets (about 14 g). Powdercorresponding to the decreased amount of powder is supplemented eachtime 100 pellets were manufactured. 10000 pellets were manufacturedunder one certain standard.

The vertical axis of FIG. 8 represents the weight of metal raw materialpowder, which corresponds to the weight of metal raw material filled inthe cartridge. In other words, the weight of metal raw material powderis expressed by the number of the pellets. The variation of weight wasmeasured for the manufactured pellet after press molding.

According to FIG. 8, it is found that when the filling amountcorresponds to the weight of 200 to 800 pellets, the pellet weight isstable. However, when the filling amount exceeds this range, thevariation gradually is increased. This is because if the filled weightis appropriate, the powder inside the cartridge is appropriately stirreddue to the level striking operation and powders are filled in thethrough hole of the press die while the particle distribution of thepowder body is maintained.

Embodiment 6

Embodiment 6 refers to a manufacturing method in which the heatingtemperature of the raw material powder at the time of press molding ismade to be in the certain range. In order to enhance the stirring effectof the raw material powder inside the cartridge and to reduce thevariation of the porosity of the pellet and the weight, it is necessaryto ensure an excellent particulate flow. Fine powders adsorb thehumidity in air, so that the particulate flow becomes poor. Therefore,the fine powders are preferably heated at temperatures in the range of50 to 100° C. before they are filled in the press die.

If the heating temperature exceeds 100° C., platinum group/noble metal,for example tungsten, is affected by an oxidation by air, which is notpreferred for manufacturing pellets. On the other hand, if the heatingtemperature is less than 50° C., the dehumidification effect by heatingis low.

FIG. 9 shows the relationship between the temperature at which rawmaterial powder is heated and the variation of the pellet weight. Thefilling amount of raw material powder filled in a struck-level cartridgeis made to be the weight corresponding to the weight of 500 pellets. Theheating was conducted by a lamp. FIG. 9 shows that when the heatingtemperature is in the range of 50 to 100° C., the weight of the pelletis stable.

Embodiment 7

Embodiment 7 refers to a manufacturing method in which the descendingspeed of punch and the pressing time at the time of press molding aremade to be in the certain range. In the press molding, the descendingspeed of punch and the pressing time are important elements so as tocontrol the porosity distribution.

In the motion of the raw material powder inside the press die during thepress molding, the greatest motion of the powder is in the portion thatcontacts with the punch. Powder at the opposite side hardly moves.Consequently, at the powder in the vicinity of the contacting face, thepunch rubs with the press die or rubs between powders, the pressureapplied to the punch is consumed, and the pressure cannot easily betransmitted to the vicinity to the opposite side of the contacting face.Therefore, the porosity in the vicinity of the contacting face betweenthe punch and powder is low and the porosity of the opposite side ishigh.

As mentioned above, when the descending speed of the punch is increased,the incline of the porosity distribution inside the pellet is observedin the direction to which the press pressure is applied. In other words,the porosity difference between the electron emitting face and theopposite face is increased. On the contrary, if the descending speed isreduced, the press can be conducted smoothly while the friction of theraw material powder in the die is inhibited, so that more uniformporosity distribution can be obtained.

Furthermore, as the pressing time is longer, the pressure is liable tobe applied uniformly to the entire raw material powder. On the contrary,when the press molding is conducted for a short time, the pressure isapplied non-uniformly, and the porosity difference is increased betweenthe electron emitting face and the opposite face.

The measurement results of the porosity difference (vol. %) are shown inTable 2. Herein, the descending speed of the punch and the pressingtimes are respectively changed and are combined.

TABLE 2 Pressing Descending speed (cm/s) time(s) 0.2 0.5 1 3 5 7 0.2 1020 25 35 40 40 0.5 4 10 20 35 40 40 1 3 8 13 29 33 40 3 3 5 10 25 30 357 2 5 7 18 25 30 10 2 5 6 16 23 25

According to Table 2, if the descending speed is selected in the rangeof 0.5 to 5 cm/s, and the pressing time is selected in the range of 1 to7 seconds, the porosity distribution can be controlled freely. Thepressing time that is more than 7 seconds still may be excellent but notsuitable for the mass production.

As mentioned above, the average porosity all over the pellet can beindependently controlled by adjusting the press pressure. Therefore, thepellet of the present invention easily can be manufactured by a usualprocess, wherein raw material powder having a different particledistribution is not used, and molding in multilayers is not needed.

Embodiment 8

Embodiment 8 refers to a manufacturing method in which the averageporosity of the porous substrate after press molding and the averageporosity of the pellet after sintering are in a certain range.

In order to stabilize the impregnation of the electron emittingmaterials into the pellet, the continuity of the porosity, besides theporosity of the pellet, is important element. In other words, it isimportant to reduce pores that are not connected to an opening of thepellet surface and to reduce closed pores that are not impregnated withelectron emitting materials.

Furthermore, in order to ensure the mass productivity of pellets,sufficient mechanical strength is necessary.

FIG. 10 is a graph showing the relationship between the average porosityof a porous substrate after press molding and the impregnation amount ofthe electron emitting materials and the relationship between the averageporosity of a porous substrate after press molding and the fracture rateof the pellet. Lines 12 to 14 show the relationship between the averageporosity D (vol. %) of the porous substrate after press molding and theamount of impregnation of electron emitting material, in a case wherethe average porosity d (vol. %) of the pellet after sintering is changedin the range of 10 to 30 vol. %. The left vertical axis shows therelative value of the amount of impregnation per pellet. The amount ofimpregnation is made to be 1 when the average porosity d after sinteringis 20 vol. % and the average porosity D after press molding is 30 vol.%.

The results shown by lines 12 to 14 show that when the average porosityD exceeds the certain value, the amount of impregnation begins to lower.For example, in a line 12 where the average porosity d of the pelletafter sintering is 10 vol. %, the amount of impregnation is stable untilthe average porosity D is 30 vol. %, however, if it is more than 30 vol.%, the amount of impregnation begins to lower.

Lines 15 to 17 show the relationship between the average porosity D(vol. %) of the porous substrate after press molding and the relativevalue of the fracture rate of pellets in a case where the averageporosity d (vol. %) of the pellet after sintering is changed in therange of 10 to 30 vol. %. The right vertical axis shows the fracturerate of the pellets.

The results shown in lines 15 to 17 show that when the average porosityD exceeds the certain value, the fracture rate of the pellet becomes 0.For example, in line 15 where the average porosity d after sintering is10 vol. %, the fracture rate of the pellet is 0 when the averageporosity D is 20 vol. %.

According to the above mentioned measurement results, in order tomanufacture the pellet having a certain amount of impregnation whilemaintaining the mechanical strength and inhibiting the occurrence of theclosed pores, it is necessary that the relationship between the averageporosity D (vol. %) after press molding and the average porosity d (vol.%) after sintering is expressed in the following equation:

d+10≦D≦d+20.

The above mentioned expression of the relationship is shown in FIG. 11.Line 18 satisfies the relationship: D=d+10. Line 19 satisfies therelationship: D=d+20. Therefore, the hatched portion between the lines18 and 19 is the portion that satisfies the above mentioned expressionof the relationship. In the portion above the line 18, the mechanicalstrength is insufficient. On the other hand, in the area below the line19, the amount of impregnation is too little. For example, if the pellethaving the average porosity d of 20 vol. % is desired to be obtained,the average porosity D after press molding is preferably in the range of30 to 40 vol. %.

In this case, if the average porosity D is less than 30 vol. %, thepellet is hardly sintered, so that the mechanical strength lowersgreatly. Consequently, the pellet is fractured when it is handled. Onthe other hand, if the average porosity is more than 40 vol. %, thepellets are sintered too much. As a result, a great number of closedpores are generated, and an appropriate amount of electron emittingmaterials cannot be impregnated.

Embodiment 9

Embodiment 9 shows a manufacturing method in which the electron emittingmaterials filled in a container for impregnation are in the certainrange. In this embodiment, as a container for impregnation, thecontainer whose upper side is open, for example, a heat resistantmetallic container made of Mo and W was used. The container has the sizeof 1.5 cm length×1.5 cm width×1 cm depth. The electron emittingmaterials are filled in the container for impregnation in an amount thatchanges in the range of 200 to 20000 times as much as an optimum amountof impregnation per pellet. 100 pellets were placed thereon andimpregnated. The pellet has the average porosity of 20±1 vol. %, adiameter of 1.2 mm and the height of 0.42 mm. The 100 pellets wereclassified for weight at the precision of ±5 μg. After impregnation,extra electron emitting materials were removed and the weight wasmeasured. Thus the increased weight, namely, the impregnated weight wascalculated per pellet.

FIG. 12 is a graph showing the relationship between the amount ofelectron emitting materials filled in a container for impregnation andthe variation of the amount of impregnation to the pellet. Thehorizontal axis of FIG. 12 shows the filling amount, which is expressedby the number of the pellets. Namely, the filling amount is expressed byhow many times grater than the optimum amount of electron emittingmaterial in the container that necessary for one pellet (hereafter,“filling amount” will be used for an abbreviation).

According to FIG. 12, if the filling amount is less than 1000 times,pellets that are not sufficiently impregnated are generated. This isbecause some substrates are not wetted on the whole surface of theporous substrate when the electron emitting materials are melted. Whenthe filling amount is in the range of 1000 to 10000 times, the amount ofan impregnation per pellet is nearly saturated, showing the optimalamount of impregnation.

When the filling amount exceeded 10000 times, the average amount ofimpregnation was decreased. This is because a great amount of gas isgenerated when the electron emitting materials are melted and preventsthe electron emitting materials from entering the pore of the substrate.Furthermore, in a case where the bottom area of the container isincreased, when the pellets are proportionally increased in accordancewith the rate, the almost similar results can be obtained. From theabove mentioned results, it is preferable that the filling amount is inthe range of 1000 to 10000 times.

Moreover, as mentioned above, the filling amount is expressed by theweight per pellet. In this embodiment, since 100 pellets are placed inthe container for impregnation, when the above mentioned filling amountis expressed by the value corresponding to the whole pellets located ina container for impregnation, the preferable range of weight of electronemitting material is in the range of 10 to 100 times.

Embodiment 10

Embodiment 10 refers to a method for locating pellets on the containersfor impregnation. In the method, the pellets are located in such amanner that the entire surface of the pellet contacts with the electronemitting materials at the times of impregnation. In this embodiment, thefollowing experiments were carried out. The filling amount of theelectron emitting materials was set to 3000 times, which is thepreferable range shown in Embodiment 9. The impregnation was conductedin the following 4 kinds of pellet locations; a to d. FIG. 13(B) showsthe location relationship of a container for impregnation 20, pellets 21and electron emitting material 22, respectively in a case of a to d.

In a, 100 pellets were set in the same level in one stage on the bottomof the container for impregnation, and electron emitting material isfilled on the pellets. In this location, the cylindrical bottom face ofthe pellets contact with the container for impregnation.

In b, 50 pellets per stage were set in two stages on the bottom of thecontainer for impregnation, and electron emitting material is filled onthe pellets. In this location, the cylindrical upper face of the pelletof the first stage contacts with the cylindrical bottom face of thepellet of the second stage. The cylindrical bottom face of the pellet ofthe first stage contacts with the bottom area of the container.

In c, electron emitting material is filled in the container forimpregnation in a half amount by making the depth constant, then 100pellets are set in the same level in one stage on the electron emittingmaterial, and then the rest of the electron emitting material isuniformly filled by making the depth constant. In this location, theentire surface of the pellet contacts with the electron emittingmaterials.

In d, whole amount of electron emitting materials is placed in thecontainer for impregnation by making the depth constant and 100 pelletsare set in the same level in one stage. In this location, thecylindrical upper face of the pellet contacts with space.

FIG. 13(A) shows the relationship between the above mentioned locationsand the amount of impregnation to the pellet. The horizontal axes a to dcorrespond to the above mentioned locations a to d.

In the location of the pellet in a and b, a few deficiencies in theimpregnation occurred. In c and d, the amount of impregnation wasexcellent. This shows that unless the entire surface of the pellet iscovered with electron emission materials, the amount of impregnation isinsufficient. Moreover, in a case of d, in the state shown in FIG.13(B), the entire surface of the pellet is not covered with electronemitting materials. However, as the electron emitting materials aremelted, the pellets sink down in the electron emitting materials due totheir weight, the whole surface is naturally covered with electronemitting material. In other words, it is an important condition forstable impregnation that the entire surface of the pellet is coveredwith electron emitting materials when the electron emitting materialsare melted.

Embodiment 11

Embodiment 11 refers to a method for removing extra electron emittingmaterials attached to the pellet at the time of the impregnation. Extraemitting materials are physically removed by means of balls forgrinding.

In this embodiment, the pellets impregnated under the optimum conditionby the method of Embodiment 10 were used. These pellets were placed inthe glass container having a volume of 100 cm³ along with, for example,10 alumina balls having a diameter of φ=5 mm, and were subjected toshaking for 5 minutes to 1 hour. Then, the pellets were subjected toultrasonic cleaning in ion exchanged water for 5 minutes, and dried invacuum. The relationship between the shaking time and the fracture rateof the pellets at this time is shown in the following Table 3.

TABLE 3 Com. Com. Com. Com. Ex. 1 Ex. 2 Ex. 1 Ex. 2 Ex. 3 Ex. 3 Ex. 4Shaking time (minute) 0 0 5 15 30 60  120 Ultrasonic cleaning 5 60  5 55 5  5 time (minute) Fracture rate (%) 0 0 0 0.2 0.3 1  3 Com. Ex.:Comparative Example Ex.: Example

Table 3 shows that in the pellet that was subjected to a shaking for 60minutes or more (Comparative Example 3 and 4), the fracture rate of thepellets is rapidly increased.

Furthermore, the amounts of impregnation to the pellets in ComparativeExamples 1 to 4 and Examples 1 to 3 in Table 3 are shown in FIG. 14.FIG. 14 shows that the variation of the amount of impregnation to thepellet is minimum in Example 2 (the shaking time is 15 minutes). Sincethis variation reflects the attaching level of extra electron emittingmaterials, the pellet is excellent as this variation is smaller. Thevariation is small when the shaking time is 60 minutes or more(Comparative Examples 3 and 4), however, the fracture rate of thepellets is increased as mentioned above.

According to the results of the Comparative Examples 1 and 2 (no shakingwas conducted), the variation per pellet is little decreased even if thecleaning time is increased when only the ultrasonic cleaning isconducted. This shows that effective electron emitting materials inpores, as well as extra electron emitting material, are removed overtime. In addition, it is found that this method requires an absolutelylong time of treatment. Consequently, it is not suitable for massproduction.

Moreover, the conditions of the shaking or rolling, etc. freely can bechanged by selecting the number of balls, size, volume of container,amount of the pellet to be treated, times, number of vibration frequencyand amplitude of shaking, and rolling speed.

As mentioned above, in each embodiment, tungsten (W) was used as oneexample of the material constituting the pellet. However, the materialis not limited to this alone, it may be the high melting point metals,for example, osmium (Os), ruthenium (Ru), iridium (Ir), rhenium (Re),tantalum (Ta), molybdenum (Mo), etc., an alloy comprising these metals,or materials based on these metals and comprising a small amount ofadditives.

Furthermore, in the above mentioned embodiments, the mixture comprisingbarium carbonate (BaCO₃), calcium carbonate (CaCO₃), aluminum oxide(Al₂O₃) in a mole ration of 4:1:1 was used as one example of electronemitting materials. The electron emitting material is not limited tothis alone. The mixture in which the above mole ratio is changed may beused, and these mixtures in which a few amount of additives aredispersed may be used. Furthermore, instead of barium carbonate, bariumoxide (BaO) may be used; and instead of calcium carbonate, calcium oxide(CaO) may be used.

Finally, it is understood that the invention may be embodied in otherspecific forms without departing from the spirit or essentialcharacteristics thereof. The embodiments disclosed in this applicationare to be considered in all respects as illustrative and notrestrictive, so that the scope of the invention being indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are intended to be embraced therein.

What is claimed is:
 1. An impregnated cathode having a cathode pellet inwhich a pore portion of a sintered body of porous metal is impregnatedwith electron emitting material, wherein the sintered body of porousmetal has a single layer structure and the porosity of said sinteredbody of porous metal is continuously increased as the distance in adepth direction from an electron emitting face is increased.
 2. Theimpregnated cathode according to claim 1, wherein the porosity of anelectron emitting face of said sintered body of porous metal is in therange of 12.5 to 25 volume %; the porosity difference between theporosity of a vicinity of said electron emitting face and the porosityof a vicinity of the face opposite to said electron emitting face is inthe range of 5 to 25 volume %; and the porosity of the side opposite tosaid electron emitting face is less than 40 volume %.
 3. The impregnatedcathode according to claim 1, wherein the surface roughness of theelectron emitting face of said cathode pellet is in the range of 5 to 20μm for the maximum height.